The present invention generally relates to the automatic dilution or dispensing of fluids, more particularly to an apparatus and a method for diluting or dispensing fluids in a manner that includes automatic compensation for the non-linearity of aspirated fluid volumes including non-linearity due to unavoidable variations in cross-sectional area throughout the length of precision ground pistons of the aspirating syringe assemblies of diluter/dispenser devices or variations in cylinder bores of classical syringe designs which employ a plunger drive against the cylinder internal diameter. In connection with the compensating diluting or dispensing apparatus and method according to this invention, each piston and cylinder, or syringe, assembly is controlled by a drive assembly that compensates for deviations from the ideal, linear aspiration volume scheme for a syringe assembly of that size. The drive assembly follows an appropriate program of compensation factors for selectively adjusting the extent of movement of the piston of that particular syringe assembly in order to provide volume displacements that exhibit substantially enhanced conformity with the ideal, linear volume displacement scheme expected of the particular syringe, limited only by its inherent precision.
Laboratory test devices for performing qualitative or quantitative analyses of fluids, such as blood cell counts or hemoglobin determination on blood samples, typically require substantial dilution of the sample fluids in order to minimize the size of sample needed or in order to satisfy the requirements of laboratory procedures such as those involving colorimetric analysis. Automatic diluters, which may be free standing or a component of a more comprehensive instrument, are known and available for carrying out automatic dilution or automatic dispensing of diluents or reagents.
An extremely important consideration and objective of the various currently available automatic diluters is that a diluter must be as accurate and precise as possible so as to minimize deviation between a volume of fluid that is actually aspirated or dispensed when compared with the volume that the automatic diluter indicates has been aspirated or dispensed. A minute deviation between the actual volume and the expected volume (the volume that the device indicates has been aspirated or dispensed) typically will seriously decrease the reliability of the particular laboratory analysis being performed. Previously known automatic diluter/dispenser devices have been reasonably successful in providing for the movement of fluids in an accurate and precise manner. One such important advance in this regard is described in U.S. Pat. No. 3,655,094, incorporated by reference hereinto, which includes the use of precision piston and cylinder, or syringe, assemblies. This device is illustrative of automatic diluters that include means to ensure piston movement that is especially responsive so as to all but entirely eliminate inaccuracies that are caused by imprecise movement of each piston and of its drive components such as stepper motors, gears and drive spindles.
Even though great strides have been made in improving the accuracy and precision of automatic diluter/dispenser devices, inaccuracies throughout their operating ranges (variable versus fixed volume) still present a problem. Although the syringe assemblies incorporated into these devices are precision manufactured in order to meet extremely tight tolerances, the fact remains that known manufacturing techniques are incapable of removing all non-uniformities. The cross-sectional area of a precision manufactured piston will vary somewhat throughout the length of the piston or the length of the cylinder bore, which variations will cause non-linearity of volumes aspirated or dispensed with a syringe assembly incorporating such a piston. Inaccuracies of this type are especially critical when the volume of fluid being aspirated or dispensed is a small percentage of the total volume of the particular syringe. Further inaccuracies can develop due to very minute changes in the location or configuration of seals within piston and cylinder or syringe assemblies which tend to develop during normal operation after extensive use. Another disadvantage which can become evident upon extended use is the need to lubricate these syringe seals to prevent sticking and undesirable noise.
The present invention recognizes these problems and substantially eliminates their detrimental effects. An improved piston stroke regulating assembly is provided which includes a microprocessor controlled drive assembly that automatically compensates for the non-linearity of aspirated volumes that is exhibited by the particular syringe when it is used without compensating for its non-linearity. Corrections are automatically made so that the stroke length needed to aspirate a particular volume of fluid is increased or decreased by an amount such that the volume actually aspirated at virtually any location along the piston is substantially identical to the rated or expected volume at that location along the piston. Without this modified stroke regulating assembly, the non-linearity of the piston and cylinder or syringe assembly will result in piston movement that may be too great at one volume or volumes, while being too little at another volume or volumes. Modification schemes are developed in this regard by comparing actual volumes aspirated by a particular syringe assembly with expected volumes at a plurality of aspirating locations in order to develop a set of modification factors that are used to make the necessary adjustments to the microprocessor control assembly that will control its movement when it is used to aspirate and dispense fluids. Additionally, each piston and cylinder or syringe assembly is preferably provided with a seal assembly for the movement interface between the piston and the cylinder, which seal assembly provides a preferably lubricious sealing surface that is substantially rigid and stationary and that cylindrically overlies the surface of the piston.
It is accordingly a general object of the present invention to provide an improved automatic diluter/dispenser device and method.
Another object of the present invention is to provide a precision diluter/dispenser apparatus and method which includes a syringe stroke regulating assembly that corrects for the non-linearity of aspirated volumes that is experienced by the particular syringe piston and cylinder assembly in order to, in effect, remove bias error, thereby assuring that volumetric accuracy approximates the inherent precision of the machine and calibration system.
Another object of this invention is to provide an improved diluter/dispenser apparatus having enhanced long-life seal assemblies that avoid distortion which can lead to inaccurate aspiration.
These and other objects, features and advantages of this invention will be more clearly appreciated and understood through a consideration of the following detailed description.